Disc conversion dragging bad

[YungICY]

I need some help with a rear disc conversion I did on my truck.
Truck is an 85 using factory manual master cylinder, wilwood proportioning valve mounted next to the master cylinder, new hardline from master cylinder to rear to new soft line to a ruffstuff disc conversion caliper.
The rear brakes are dragging bad. If I crack open the bleeders after driving I get a little spurt of fluid under pressure. Lowering the pressure via the proportioning valve seems to help but doesn’t alleviate the issue. Sitting in my driveway in neutral I can’t roll the truck
Is there a residual valve built into these masters for the original drums?

 

[pentode]

That's a master cylinder that was originally made from rear drums? If so I can't say for certain, someone here will chime in soon who knows for sure, but I'd bet money that's your problem. Lots of drum brake masters have check valves built in. Would probably be right near the lines that go to the rear brakes. You're on the right track for sure.

edit: if this is in fact the problem, I wonder if you can just delete that check valve. I can't see why not right off hand given that you have a proportioning valve.

 

[bobbywalter]

usually they work ok. but if i remember right...might be wrong....the early trucks up to 86 or so did have a pressure valve that may keep to much pressure. they are the all metal old school units.

 

[bobbywalter]

they were manual brake ones too that were an issue.

 

[YungICY]

That’s what I’m working with is the old all metal cast unit. There definitely isn’t any valves external but I was doing some reading and several different forums ford based and otherwise had made mention of internal valves.

 

[bobbywalter]

the fix was to use the later units.

 

[YungICY]

Basically anything with the plastic reservoir? I’ll start doing some digging and comparing.

 

[pentode]

I'm guessing if the fix was to use newer units, maybe it's not possible to remove the check valve. But I'd be curious to find out either way.

 

[bobbywalter]

yeah. i used different units from different years with varied results. the wiring is different for the brake light and cruise control ect from the later explorers....and line adaptations ect.

i have hydroboost now because i have a diesel...so i have a all gm system.

 

[alwaysFlOoReD]

I remember reading (long time ago...) that the residual valve was about 8psi for drums, and should be about 2psi for disc. For some reason I thought the valve was on the frame rail under drivers foot or so.

 

[YungICY]

I’ll do some more reading and see if I can find more sources on an internal valve.

Pentode, if I can confirm it’s the master maybe I’ll dissect mine and report back.
FlOoReD, there is an H junction block on the frame rail but (and I could be wrong) I’m pretty sure that is just to illuminate a light on the dash when there’s a difference in pressure between the front and rear ie, one of the reservoirs is empty.

 

[alwaysFlOoReD]

I’ll do some more reading and see if I can find more sources on an internal valve.

Pentode, if I can confirm it’s the master maybe I’ll dissect mine and report back.
FlOoReD, there is an H junction block on the frame rail but (and I could be wrong) I’m pretty sure that is just to illuminate a light on the dash when there’s a difference in pressure between the front and rear ie, one of the reservoirs is empty.

That does ring a bell too.

 

[bobbywalter]

I’ll do some more reading and see if I can find more sources on an internal valve.

Pentode, if I can confirm it’s the master maybe I’ll dissect mine and report back.
FlOoReD, there is an H junction block on the frame rail but (and I could be wrong) I’m pretty sure that is just to illuminate a light on the dash when there’s a difference in pressure between the front and rear ie, one of the reservoirs is empty.

that can have a valve in it

 

[JoshT]

IIRC I've also read about the residual valve being in the master cylinder port. People would run a screw into it and pull it out when doing a rear disc conversion. Don't recall the application, and I could be thinking about something else entirely.

 

[Uncle Gump]

I’ll do some more reading and see if I can find more sources on an internal valve.

Pentode, if I can confirm it’s the master maybe I’ll dissect mine and report back.
FlOoReD, there is an H junction block on the frame rail but (and I could be wrong) I’m pretty sure that is just to illuminate a light on the dash when there’s a difference in pressure between the front and rear ie, one of the reservoirs is empty.

That's the proportioning valve... it likely has the residual valve too. It does also turn on the brake indicator... not when a reservoir is empty but when front or rear brakes hydraulically fail and the uneven pressure trips the brake light switch in either direction.